The prior art discloses various methods for the synthesis and generation of three dimensional sound effects. All of these methods involve the use and synthesis of head-related transfer functions (HRTFs). These HRTFs define exactly the acoustic filtering characteristics of an individual's external auditory periphery, consisting primarily of the torso, shoulders, head, neck, and external ears, (hereinafter referred to as the external ‘auditory periphery’). The HRTFs are dependent on the precise shape and geometry of the external auditory periphery. As this varies from individual to individual, each individual and each spatial direction around the individual requires its own unique HRTF for the left ear and for the right ear in order to accurately synthesis virtual auditory space (VAS), which refers to the electronic synthesis of spatial hearing in an artificial acoustic environment.
The prior art specifies HRTFs in both the time domain and in the frequency domain. Time domain descriptions of the HRTFs take the form of coefficients for a finite impulse response (FIR) filter, coefficients for an infinite impulse response (IIR) filter, or as sound samples. Frequency domain descriptions of HRTFs take the form of a complex-valued frequency response, a magnitude frequency response, or as frequency equalisation weights. The prior art also uses principal component analysis to compress the representation of HRTFs at single locations. The principal component analysis can be applied to either the time domain or the frequency domain representation of the HRTFs.
Some of the prior methods for the synthesis and generation of three dimensional sound effects do not use customised HRTFs, but use the same approximate HRTFs for all individuals. Approximate HRTFs are derived from a population average, from an acoustic mannequin or from an acoustic model of the acoustic filtering of the external auditory periphery. More particularly, some methods use a statistical means to compute approximate HRTFs as the population average of a set of several individuals' HRTFs, some methods use the approximate HRTFs provided by an acoustic mannequin, such as the KEMAR mannequin, the Brüel-Kjær mannequin, the Head Acoustic mannequin, or the like, while some methods determine an approximate HRTF from a standard set of parameter values for an acoustic model of the external auditory periphery.
Some of the prior methods for customising the process of synthesis and generation of three dimensional sound effects for individual listeners involve sending a person to an acoustic laboratory with the equipment required to acoustically record the HRTFs. A variation of this is taking a physical mould of the person's ears and then attaching the ear moulds to an acoustic mannequin and acoustically recording the HRTFs of the mannequin combined with the new ear moulds in the appropriate laboratory. The acoustic mannequins that are frequently used are the KEMAR mannequin, the Brüel-Kjær mannequin, or the Head Acoustic mannequin.
Other prior art methods for customising the process of synthesis and generation of three dimensional sound effects for individuals attempt to avoid the difficult acoustical measurements which are costly in both time and equipment. These methods may involve, for example, taking an optical scan of a person's head and then using image processing to produce an image suitable for computer simulation of acoustic wave propagation. Still other methods involve using a database of acoustically recorded HRTFs for a set of known listeners, searching the database for the HRTF that best matches an unknown listener by playing test sounds filtered with the HRTFs in the database to the unknown listener and then asking the unknown listener questions about the quality and location of the synthesized sound Another method, which uses a database of HRTFs, involves scaling the frequency axis of the HRTFs in the database in order to customise a set of HRTFs for the individual listener. It has not yet been reported whether there exists a means for determining the scaling parameter. Yet another method involves generating a set of HRTFs using electro-acoustic simulation of the individual's external auditory periphery.
All of these prior art methods have disadvantages associated with them, with the primary disadvantage being that none of them, except for the acoustical measurements in the laboratory and the computer simulation of the acoustic wave equation based on an optical image, provide a reliable and workable method with which to relate the physical morphology of the individual listener's external auditory periphery to a usable set of HRTFs with any controllable degree of accuracy. The acoustical measurement process is cumbersome, costly and inefficient. The computer simulation of the acoustic wave equation requires an optical image which requires such high definition that a mould of the ear usually has to be taken for the optical imaging. This usually ends up costing as much in time and equipment as the acoustical measurements in the first place. All of the other methods which do not involve acoustical measurements have uncontrollable errors
Certain of the prior art methods do not produce customised HRTFs. Instead they use the same approximate HRTFs for all individual listeners. The first method using a database of HRTFs referred to above, while producing a set of “best match” HRTFs, requires the individual listener to listen to many different sounds filtered with other individual's HRTFs in the database in order to find the best match. This is time-consuming and imprecise. The second method which also uses a database of HRTFs referred to above, does not have a direct means to determine which HRTFs in the database should be used to apply a scaling of the frequency axis or to what value the scaling parameter should be set. An additional disadvantage for all of the known methods which use a database of HRTFs is that they do not have any procedure for improving the customisation to any reasonable degree of accuracy. Furthermore, the size of the database would have to be inordinately large to achieve high-fidelity. The electro-acoustic method is difficult because it requires estimating and setting parameters for electronic circuits such as resonators, filters, adders, and time-delay circuits and is not reliable.
Certain of the prior art methods described above have disadvantages in that they do not provide a single means for easily producing varying degrees of customisation of HRTFs for individual listeners. Such a varying degree of customisation is likely to be valuable in developing and applying VAS technology to its different areas of application.
Other of the prior art methods described above have disadvantages in their data storage and compression of HRTF data across a population of people. A single device suitable for use by many different individual listeners typically requires a large database of HRTFs.
Further, some of the prior art methods described above have disadvantages in the inefficiency of their searching procedure for finding the best set of customised HRTFs for an individual listener.
Still other of the prior art methods described above have disadvantages in not having a direct procedure for producing customised HRTFs that do not require using or searching a database of HRTFs.
Finally, some of the prior art methods described above have disadvantages in that they do not have a means to reduce the amount of acoustical or optical measurements required for customising a set of HRTFs for an individual listener, while maintaining high-fidelity in the HRTFs. Such a reduction in the amount of measurements required is valuable because it reduces the amount of time required for active participation of the individual listener during the acoustical or optical measurement process. Some of the prior art methods using acoustical measurements do not have a means for systematically reducing the number of locations at which HRTFs are to be measured. Some of the prior art methods using optical measurement techniques do not have a means for reducing the details of the optical image because they must rely on a computer simulation of acoustic wave propagation.